This invention relates to a method and an apparatus for recording and reproducing information and in particular to a method and an apparatus for reproducing information suitable to be used in an optical disk device for demodulating recorded information by using the rise and fall of reproduction signal pulses as data.
The method of demodulating data by detecting the rise and fall of the waveform of the reproduction signal can be utilized for a DAD (digital audio disk). The principle of the data demodulation is described e.g. in "Introduction to Video Disk and DAD" by Iwamura (Corona Publishing Co., Japan) pp. 212-215. The demodulation is performed by detecting variation points in the waveform of the reproduction signal (modulated wave), i.e. the lead-edge and the tail-edge of the waveform, and producing a reproductive window therefrom, in order to obtain reproduced data. In the method utilized for DAD, it is the condition, under which correct demodulation is possible, that the width of the reproductive window is T/2, where T denotes a data interval, and that pulses representing variation points are located in a region of .+-.0.25 T. Consequently, when a zero cross point is varied by noise, distortion, jitter, etc. and it is outside of the reproductive window, an error is produced. Although it is conceivable also for the write-once type optical disk to form data, using the lead-edge and the tail-edge, for the write-once type, since the object disk is directly irradiated with laser light pulses, the thermal energy of which alters locally the recording medium, so that the data is registered in the medium, and since the position of the lead-edge and the tail-edge of formed recording domains (pits or magnetic domains) are strongly influenced by characteristics of the recording medium and jitter, it is apt to be shifted indefinitely. Therefore, it is important to correct edge shift of the lead-edge and the tail-edge during recording by some method.